Not Logged In

virtualenv 1.10

What It Does

virtualenv is a tool to create isolated Python environments.

The basic problem being addressed is one of dependencies and versions,
and indirectly permissions. Imagine you have an application that
needs version 1 of LibFoo, but another application requires version
2. How can you use both these applications? If you install
everything into /usr/lib/python2.7/site-packages (or whatever your
platform’s standard location is), it’s easy to end up in a situation
where you unintentionally upgrade an application that shouldn’t be
upgraded.

Or more generally, what if you want to install an application and
leave it be? If an application works, any change in its libraries or
the versions of those libraries can break the application.

Also, what if you can’t install packages into the global
site-packages directory? For instance, on a shared host.

In all these cases, virtualenv can help you. It creates an
environment that has its own installation directories, that doesn’t
share libraries with other virtualenv environments (and optionally
doesn’t access the globally installed libraries either).

Installation

Warning

We advise installing virtualenv-1.9 or greater. Prior to version 1.9, the
pip included in virtualenv did not not download from PyPI over SSL.

Warning

When using pip to install virtualenv, we advise using pip 1.3 or greater.
Prior to version 1.3, pip did not not download from PyPI over SSL.

Warning

We advise against using easy_install to install virtualenv when using
setuptools < 0.9.7, because easy_install didn’t download from PyPI over SSL
and was broken in some subtle ways.

To install globally with pip (if you have pip 1.3 or greater installed globally):

The virtualenv.py script is not supported if run without the
necessary pip/setuptools/virtualenv distributions available locally. All
of the installation methods above include a virtualenv_support
directory alongside virtualenv.py which contains a complete set of
pip and setuptools distributions, and so are fully supported.

Usage

The basic usage is:

$ virtualenv ENV

This creates ENV/lib/pythonX.X/site-packages, where any libraries you
install will go. It also creates ENV/bin/python, which is a Python
interpreter that uses this environment. Anytime you use that interpreter
(including when a script has #!/path/to/ENV/bin/python in it) the libraries
in that environment will be used.

Virtualenv (<1.10) used to provide a --distribute option to use the
setuptools fork Distribute. Since Distribute has been merged back into
setuptools this option is now no-op, it will always use the improved
setuptools releases.

A new virtualenv also includes the pip
installer, so you can use ENV/bin/pip to install additional packages into
the environment.

activate script

In a newly created virtualenv there will be a bin/activate shell
script. For Windows systems, activation scripts are provided for CMD.exe
and Powershell.

On Posix systems you can do:

$ source bin/activate

This will change your $PATH so its first entry is the virtualenv’s
bin/ directory. (You have to use source because it changes your
shell environment in-place.) This is all it does; it’s purely a
convenience. If you directly run a script or the python interpreter
from the virtualenv’s bin/ directory (e.g. path/to/env/bin/pip
or /path/to/env/bin/python script.py) there’s no need for
activation.

After activating an environment you can use the function deactivate to
undo the changes to your $PATH.

The activate script will also modify your shell prompt to indicate
which environment is currently active. You can disable this behavior,
which can be useful if you have your own custom prompt that already
displays the active environment name. To do so, set the
VIRTUAL_ENV_DISABLE_PROMPT environment variable to any non-empty
value before running the activate script.

On Windows you just do:

> \path\to\env\Scripts\activate

And type deactivate to undo the changes.

Based on your active shell (CMD.exe or Powershell.exe), Windows will use
either activate.bat or activate.ps1 (as appropriate) to activate the
virtual environment. If using Powershell, see the notes about code signing
below.

Note

If using Powershell, the activate script is subject to the
execution policies on the system. By default on Windows 7, the system’s
excution policy is set to Restricted, meaning no scripts like the
activate script are allowed to be executed. But that can’t stop us
from changing that slightly to allow it to be executed.

In order to use the script, you have to relax your system’s execution
policy to AllSigned, meaning all scripts on the system must be
digitally signed to be executed. Since the virtualenv activation
script is signed by one of the authors (Jannis Leidel) this level of
the execution policy suffices. As an administrator run:

PS C:\> Set-ExecutionPolicy AllSigned

Then you’ll be asked to trust the signer, when executing the script.
You will be prompted with the following:

PS C:\> virtualenv .\foo
New python executable in C:\foo\Scripts\python.exe
Installing setuptools................done.
Installing pip...................done.
PS C:\> .\foo\scripts\activate
Do you want to run software from this untrusted publisher?
File C:\foo\scripts\activate.ps1 is published by E=jannis@leidel.info,
CN=Jannis Leidel, L=Berlin, S=Berlin, C=DE, Description=581796-Gh7xfJxkxQSIO4E0
and is not trusted on your system. Only run scripts from trusted publishers.
[V] Never run [D] Do not run [R] Run once [A] Always run [?] Help
(default is "D"):A
(foo) PS C:\>

If you select [A] Always Run, the certificate will be added to the
Trusted Publishers of your user account, and will be trusted in this
user’s context henceforth. If you select [R] Run Once, the script will
be run, but you will be prometed on a subsequent invocation. Advanced users
can add the signer’s certificate to the Trusted Publishers of the Computer
account to apply to all users (though this technique is out of scope of this
document).

Alternatively, you may relax the system execution policy to allow running
of local scripts without verifying the code signature using the following:

PS C:\> Set-ExecutionPolicy RemoteSigned

Since the activate.ps1 script is generated locally for each virtualenv,
it is not considered a remote script and can then be executed.

The --system-site-packages Option

This can be used if you have control over the global site-packages directory,
and you want to depend on the packages there. If you want isolation from the
global system, do not use this flag.

Environment variables and configuration files

virtualenv can not only be configured by passing command line options such as
--python but also by two other means:

Environment variables

Each command line option is automatically used to look for environment
variables with the name format VIRTUALENV_<UPPER_NAME>. That means
the name of the command line options are capitalized and have dashes
('-') replaced with underscores ('_').

For example, to automatically use a custom Python binary instead of the
one virtualenv is run with you can also set an environment variable:

virtualenv also looks for a standard ini config file. On Unix and Mac OS X
that’s $HOME/.virtualenv/virtualenv.ini and on Windows, it’s
%APPDATA%\virtualenv\virtualenv.ini.

The names of the settings are derived from the long command line option,
e.g. the option --python would look like this:

[virtualenv]
python = /opt/python-3.3/bin/python

Appending options like --extra-search-dir can be written on multiple
lines:

[virtualenv]
extra-search-dir =
/path/to/dists
/path/to/other/dists

Please have a look at the output of virtualenv --help for a full list
of supported options.

Windows Notes

Some paths within the virtualenv are slightly different on Windows: scripts and
executables on Windows go in ENV\Scripts\ instead of ENV/bin/ and
libraries go in ENV\Lib\ rather than ENV/lib/.

To create a virtualenv under a path with spaces in it on Windows, you’ll need
the win32api library installed.

PyPy Support

Beginning with virtualenv version 1.5 PyPy is
supported. To use PyPy 1.4 or 1.4.1, you need a version of virtualenv >= 1.5.
To use PyPy 1.5, you need a version of virtualenv >= 1.6.1.

Creating Your Own Bootstrap Scripts

While this creates an environment, it doesn’t put anything into the
environment. Developers may find it useful to distribute a script
that sets up a particular environment, for example a script that
installs a particular web application.

To create a script like this, call
virtualenv.create_bootstrap_script(extra_text), and write the
result to your new bootstrapping script. Here’s the documentation
from the docstring:

Creates a bootstrap script, which is like this script but with
extend_parser, adjust_options, and after_install hooks.

This returns a string that (written to disk of course) can be used
as a bootstrap script with your own customizations. The script
will be the standard virtualenv.py script, with your extra text
added (your extra text should be Python code).

If you include these functions, they will be called:

extend_parser(optparse_parser):

You can add or remove options from the parser here.

adjust_options(options, args):

You can change options here, or change the args (if you accept
different kinds of arguments, be sure you modify args so it is
only [DEST_DIR]).

after_install(options, home_dir):

After everything is installed, this function is called. This
is probably the function you are most likely to use. An
example would be:

Using Virtualenv without bin/python

Sometimes you can’t or don’t want to use the Python interpreter
created by the virtualenv. For instance, in a mod_python or mod_wsgi
environment, there is only one interpreter.

Luckily, it’s easy. You must use the custom Python interpreter to
install libraries. But to use libraries, you just have to be sure
the path is correct. A script is available to correct the path. You
can setup the environment like:

This will change sys.path and even change sys.prefix, but also allow
you to use an existing interpreter. Items in your environment will show up
first on sys.path, before global items. However, global items will
always be accessible (as if the --system-site-packages flag had been used
in creating the environment, whether it was or not). Also, this cannot undo
the activation of other environments, or modules that have been imported.
You shouldn’t try to, for instance, activate an environment before a web
request; you should activate one environment as early as possible, and not
do it again in that process.

Making Environments Relocatable

Note: this option is somewhat experimental, and there are probably
caveats that have not yet been identified.

Warning

The --relocatable option currently has a number of issues,
and is not guaranteed to work in all circumstances. It is possible
that the option will be deprecated in a future version of virtualenv.

Normally environments are tied to a specific path. That means that
you cannot move an environment around or copy it to another computer.
You can fix up an environment to make it relocatable with the
command:

$ virtualenv --relocatable ENV

This will make some of the files created by setuptools use relative paths,
and will change all the scripts to use activate_this.py instead of using
the location of the Python interpreter to select the environment.

Note: scripts which have been made relocatable will only work if
the virtualenv is activated, specifically the python executable from
the virtualenv must be the first one on the system PATH. Also note that
the activate scripts are not currently made relocatable by
virtualenv --relocatable.

Note: you must run this after you’ve installed any packages into
the environment. If you make an environment relocatable, then
install a new package, you must run virtualenv --relocatable
again.

Also, this does not make your packages cross-platform. You can
move the directory around, but it can only be used on other similar
computers. Some known environmental differences that can cause
incompatibilities: a different version of Python, when one platform
uses UCS2 for its internal unicode representation and another uses
UCS4 (a compile-time option), obvious platform changes like Windows
vs. Linux, or Intel vs. ARM, and if you have libraries that bind to C
libraries on the system, if those C libraries are located somewhere
different (either different versions, or a different filesystem
layout).

If you use this flag to create an environment, currently, the
--system-site-packages option will be implied.

The --extra-search-dir option

Note

Currently, this feature only partially works for pip, and not at
all for setuptools. For details,
see Issue #327

This option allows you to provide your own versions of setuptools
and/or pip on the filesystem, and tell virtualenv to use those distributions
instead of the ones in virtualenv_support.

To use this feature, pass one or more --extra-search-dir options to
virtualenv like this:

$ virtualenv --extra-search-dir=/path/to/distributions ENV

The /path/to/distributions path should point to a directory that
contains setuptools and/or pip distributions. Setuptools distributions
must be .egg files; pip distributions should be .tar.gz source
distributions.

If no satisfactory local distributions are found, virtualenv will fail. Virtualenv will never download packages.

The distribution lookup is done in the following locations, with the most
recent version found used:

The current directory.

The directory where virtualenv.py is located.

A virtualenv_support directory relative to the directory where
virtualenv.py is located.

If the file being executed is not named virtualenv.py (i.e. is a boot
script), a virtualenv_support directory relative to wherever
virtualenv.py is actually installed.

Compare & Contrast with Alternatives

There are several alternatives that create isolated environments:

workingenv (which I do not suggest you use anymore) is the
predecessor to this library. It used the main Python interpreter,
but relied on setting $PYTHONPATH to activate the environment.
This causes problems when running Python scripts that aren’t part of
the environment (e.g., a globally installed hg or bzr). It
also conflicted a lot with Setuptools.

virtual-python
is also a predecessor to this library. It uses only symlinks, so it
couldn’t work on Windows. It also symlinks over the entire
standard library and global site-packages. As a result, it
won’t see new additions to the global site-packages.

This script only symlinks a small portion of the standard library
into the environment, and so on Windows it is feasible to simply
copy these files over. Also, it creates a new/empty
site-packages and also adds the global site-packages to the
path, so updates are tracked separately. This script also installs
Setuptools automatically, saving a step and avoiding the need for
network access.

zc.buildout doesn’t
create an isolated Python environment in the same style, but
achieves similar results through a declarative config file that sets
up scripts with very particular packages. As a declarative system,
it is somewhat easier to repeat and manage, but more difficult to
experiment with. zc.buildout includes the ability to setup
non-Python systems (e.g., a database server or an Apache instance).

I strongly recommend anyone doing application development or
deployment use one of these tools.

Contributing

Virtualenv’s release schedule is tied to pip’s – each time there’s a new pip
release, there will be a new virtualenv release that bundles the new version of
pip.

Files in the virtualenv_embedded/ subdirectory are embedded into
virtualenv.py itself as base64-encoded strings (in order to support
single-file use of virtualenv.py without installing it). If your patch
changes any file in virtualenv_embedded/, run bin/rebuild-script.py to
update the embedded version of that file in virtualenv.py; commit that and
submit it as part of your patch / pull request.

Running the tests

Virtualenv’s test suite is small and not yet at all comprehensive, but we aim
to grow it.

The easy way to run tests (handles test dependencies automatically):

$ python setup.py test

If you want to run only a selection of the tests, you’ll need to run them
directly with nose instead. Create a virtualenv, and install required
packages:

Changes & News

Warning

Python bugfix releases 2.6.8, 2.7.3, 3.1.5 and 3.2.3 include a change that
will cause “import random” to fail with “cannot import name urandom” on any
virtualenv created on a Unix host with an earlier release of Python
2.6/2.7/3.1/3.2, if the underlying system Python is upgraded. This is due to
the fact that a virtualenv uses the system Python’s standard library but
contains its own copy of the Python interpreter, so an upgrade to the system
Python results in a mismatch between the version of the Python interpreter
and the version of the standard library. It can be fixed by removing
$ENV/bin/python and re-running virtualenv on the same target directory
with the upgraded Python.

1.10 (2013-07-23)

BACKWARDS INCOMPATIBLE Dropped support for Python 2.5. The minimum
supported Python version is now Python 2.6.

BACKWARDS INCOMPATIBLE Using virtualenv.py as an isolated script
(i.e. without an associated virtualenv_support directory) is no longer
supported for security reasons and will fail with an error.

Along with this, --never-download is now always pinned to True, and
is only being maintained in the short term for backward compatibility
(Pull #412).

IMPORTANT Switched to the new setuptools (v0.9.7) which has been merged
with Distribute again and works for Python 2 and 3 with one codebase.
The --distribute and --setuptools options are now no-op.

Updated to pip 1.4.

Added support for PyPy3k

Added the option to use a version number with the -p option to get the
system copy of that Python version (Windows only)

Removed embedded ez_setup.py, distribute_setup.py and
distribute_from_egg.py files as part of switching to merged setuptools.

Fixed --relocatable to work better on Windows.

Fixed issue with readline on Windows.

1.9.1 (2013-03-08)

Updated to pip 1.3.1 that fixed a major backward incompatible change of
parsing URLs to externally hosted packages that got accidentily included
in pip 1.3.

Fixed inconsistency when having set the PYTHONDONTWRITEBYTECODE env var
with the –distribute option or the VIRTUALENV_USE_DISTRIBUTE env var.
VIRTUALENV_USE_DISTRIBUTE is now considered again as a legacy alias.

Use non-recursive symlinks to fix things up for posix_local install
scheme. Thanks michr.

Made activate script available for use with msys and cygwin on Windows.
Thanks Greg Haskins, Cliff Xuan, Jonathan Griffin and Doug Napoleone.
Fixes #176.

Fixed creation of virtualenvs on Windows when Python is not installed for
all users. Thanks Anatoly Techtonik for report and patch and Doug
Napoleone for testing and confirmation. Fixes #87.

Fixed creation of virtualenvs using -p in installs where some modules
that ought to be in the standard library (e.g. readline) are actually
installed in site-packages next to virtualenv.py. Thanks Greg Haskins
for report and fix. Fixes #167.

1.3.1

Always set [install] prefix in distutils.cfg – previously
on some platforms where a system-wide distutils.cfg was present
with a prefix setting, packages would be installed globally
(usually in /usr/local/lib/pythonX.Y/site-packages).

Sometimes Cygwin seems to leave .exe off sys.executable; a
workaround is added.

Fix --python option.

Fixed handling of Jython environments that use a
jython-complete.jar.

1.3

Update to Setuptools 0.6c9

Added an option virtualenv --relocatable EXISTING_ENV, which
will make an existing environment “relocatable” – the paths will
not be absolute in scripts, .egg-info and .pth files. This
may assist in building environments that can be moved and copied.
You have to run this after any new packages installed.

Added bin/activate_this.py, a file you can use like
execfile("path_to/activate_this.py",dict(__file__="path_to/activate_this.py")) – this will activate
the environment in place, similar to what the mod_wsgi example
does.

For Mac framework builds of Python, the site-packages directory
/Library/Python/X.Y/site-packages is added to sys.path, from
Andrea Rech.

Some platform-specific modules in Macs are added to the path now
(plat-darwin/, plat-mac/, plat-mac/lib-scriptpackages),
from Andrea Rech.

Fixed a small Bashism in the bin/activate shell script.

Added __future__ to the list of required modules, for Python
2.3. You’ll still need to backport your own subprocess module.